Powertrains that have an internal combustion engine coupled to a continuously or infinitely variable transmission are used to provide efficient drive systems. In continuously or infinitely variable transmissions (hereinafter “CVT”), the transmission ratio can be changed in a continuous manner from a maximum under drive ratio to a maximum over drive ratio. This permits the engine to be operated at either the best fuel consumption area or the best performance area.
The CVT includes a traction drive assembly commonly in the form of a full toroidal type unit or a half toroidal type unit. Both of these devices transmit power or torque from an input disk to an output disk via rollers through a traction force. The CVT generally includes two input disks, two output disks and a plurality of rollers disposed between respective input disk/output disk pairs. Each assembly of an input disk, an output disk and a roller is a variator. Each variator has a variator speed ratio (VSR) which is the ratio between the output disk speed and the input disk speed. The variator speed ratio is a function of the angle at which the roller is positioned. The rollers are rotatably supported on trunions that have a hydraulic force applied thereon to maintain the rollers in the desired position between the respective pairs of input and output disks. The hydraulic force applied to the trunions establishes the fraction force between the disks and rollers. Systems using hydraulic solenoids and similar such hydraulic controls to vary the hydraulic force may not be able to change quickly due to lag in the hydraulic systems.
Current CVT systems may use a stepper motor or other type of accurate device to adjust an actuator position in open-loop so that the desired roller position and thus the desired VSR is achieved. A controller directs the stepper motor to move a number of steps to a calculated position, thus adjusting the actuator position. This calculated position corresponds to the desired VSR. The change in the VSR leads to a change in the transmission speed so vehicle dynamics change. Using these vehicle dynamics changes along with driver input, the CVT control system determines a new desired VSR before the stepper motor is directed to move again, adjusting the actuator position. Therefore, this CVT control system has a slow response time and is very dependent on the accuracy of the stepper motor positioning to achieve the desired VSR. Accordingly, there is a need for a control system to improve the response time and accuracy of the CVT and to reduce the cost due to the required accuracy of the stepper motor.